This invention relates to VSB (vestigial sideband) signal systems and specifically to methods and apparatus for processing the sync and data symbols in digital VSB signals.
The recently adopted standards for digital VSB terrestrial signals establishes certain data and sync symbol levels. In the ATSC (Advanced Television Systems Committee) document, the data levels to be used for the 8VSB (trellis coded) and 16VSB (ATSC) transmission systems are specified. The VSB transmission system is not restricted to over-the-air (terrestrial) transmission and Zenith Electronics Corporation has specified three more modes that can be used for cable or MMDS systems. These VSB modes are identified as 8/4/2 VSB modes. The 8VSB trellis coded and 8VSB modes are identical except for their data rates. As fully discussed in the above-mentioned patent, the data levels and sync levels in the various VSB modes may be selected to bear a desired relationship to each other, which results in great simplification and reduced cost in data slicing and error correction. Unfortunately, that desired relationship is not present during the bilevel segment and frame reference sync in the standards adopted by ATSC and need not be present in VSB signals that may be used.
The Zenith 8VSB trellis coded mode also contemplates the use of a comb filter in the receiver for minimizing interference from NTSC co-channel signals. When the comb filter is active, it increases the number of data levels, which also results in a departure from the desired relationship mentioned above.
The present invention solves the problems created by the above over-the-air signals and enables a VSB receiver to process all VSB mode signals in an efficient and cost effective manner.